Labour Party deputy leader Harriet Harman said on Sky News that when she left her Camberwell and Peckham constituency tonight, people were still queuing up outside waiting to vote.

Business Secretary Lord Mandelson acknowledged on BBC News that the situation could lead to legal challenges.

“What the returning officers should have done is brought everyone in and locked the door.”

“It shows that there has been a high turn-out. We have got to make sure that all the votes are counted.”

‘Third world’

Scottish First Minister Alex Salmond branded the situation as akin to “a third world lack of democracy”.

He told BBC Scotland he could “well understand the fury of thousands of people who queued for some considerable time to be allowed to vote”.

He added: “It’s a third world lack of democracy we’re seeing.”

The commission said it was a cause for “serious concern” that many people who wanted to vote were prevented from doing so, and said there should have been sufficient resources allocated to make sure everyone was able to vote.

In a statement, the Electoral Commission said: “It is a cause for serious concern that many people who wanted to vote today were unable to do so by 10pm when polls closed.

“Each returning officer is responsible for deciding numbers of polling stations in their constituency and the numbers of electors allocated to each polling station.

“By law, polls must close at 10pm and any voter issued with a ballot paper by 10pm should be allowed time to cast it, but no ballot paper should be issued after 10pm.

“There should have been sufficient resources allocated to ensure that everyone who wished to vote was able to do so.

“The Electoral Commission will be undertaking a thorough review of what has happened in those constituencies where people have been unable to vote.”